D'Addario has great customer service. Here is my story if you care to read it. I have been playing D'Addario nylon tape wounds on my SR900 for about a year and though they still sounded good, I thought it might be time to change them, so I asked for and got a set for Christmas. After they were on, I noticed the D string sounded dull and a lot lower in volume than the other strings. The E was slightly dull too, but not too bad. I noticed that those two strings were a different color than the rest; they were an ashy grey and the other two were black. Since those strings are not available as singles on their website, I called them to see if they would sell me just the D string. I didn't get the man's name, but customer service was great. He said he would send me the D string at no charge, and after talking with me for a minute about the set, decided to send me an entire new set, no charge. They arrived the other day and I changed them out last night. The new set sounds fantastic. I tossed the original bad E and D strings and kept the A and G as spares. D'Addario didn't have to send me an entire set, that they did says to me that they go above and beyond to keep a customer happy. Thank you unnamed customer service guy and D'Addario.